This invention relates to apparatus for and method of making a flexible tape transducer.
The electret tape transducer described in U.S. Pat. No. 4,023,155 has the advantages of being flat, flexible and of providing unidirectional object detection capability so that it operates both as an active and as a passive transducer. This tape transducer, however, has a disadvantage in that proper electrical shielding requires that a conducting strip be located on the side of the inner conducting strip opposite from the moving conducting strip and be electrically connected to the latter along its marginal edges. For purposes of discussion, this transducer design can be considered as an upper portion and a lower portion with the inner conducting strip on the boundary between them. The upper portion provides the transduction function while the lower portion is inactive. However, since the basic transducer when used to detect vibrations is a charge generator and since the inactive capacitance of the lower portion is in parallel with that of the active portion, the output voltage of the transducer is reduced by the presence of the shielding strip. For example, if the active and inactive capacitances are equal, the output voltage of the transducer is reduced by 6 dB.
The inactive capacitance may be reduced to a negligible amount by sufficiently increasing the spacing between the shielding strip and the inner strip and by using as a spacer a low dielectric constant material such as plastic foam. The difficulty with this approach is that the thickness of the tape transducer is substantially increased and the flexibility and rollability of the device are substantially reduced. Both of these results are undesirable and in some cases unacceptable. Another disadvantage of this design is the requirement that the center strip have high inertia relative to that of the moving strip. This requirement is achieved by using a thick, heavy metal tape which increases the cost of the transducer.
A tape transducer which overcomes these disadvantages and, without sacrificing performance, retains shielding and the desired mechanical characteristics of flexibility and has a very thin profile is described in copending application Ser. No. 244,491 by Miller et al, assigned to the assignee of this invention. The improved transducer utilizes two active lay strips instead of one. Both insulating layers between the center strip and outer strips are poled to form electrets. Any motion of the outer strips with respect to the electrets produces an electrical signal output across the outer and inner strips. By poling the electret layers symmetrically with respect to the center strip, reduction of the spacing from either or both outer strips to the center strip produces the same polarity signal and thus the device is sensitive to pressure.
A further advantage of this improved tape transducer occurs in the radiation mode, i.e., when the tape is energized to produce sonic or ultrasonic waves. When a voltage is applied between the center and outer strips, forces are produced on both sides of the center strip that the symmetrical with respect to it. Thus the electrically reactive force enables the use of a thinner, lighter center strip with savings of cost and weight. Moreover the need for dielectric foam or the like for spacing one of the outer strips from the center strip is eliminated thus providing a thin flexible transducer that is easy to handle and occupies minimum space.
This invention is directed to apparatus for and method of making this improved tape transducer.